Multilayer CVD graphene electrodes using a transfer-free process for the next generation of optically transparent and MRI-compatible neural interfaces

Multimodal platforms combining electrical neural recording and stimulation, optogenetics, optical imaging, and magnetic resonance (MRI) imaging are emerging as a promising platform to enhance the depth of characterization in neuroscientific research. Electrically conductive, optically transparent, and MRI-compatible electrodes can optimally combine all modalities. Graphene as a suitable electrode candidate material can be grown via chemical vapor deposition (CVD) processes and sandwiched between transparent biocompatible polymers. However, due to the high graphene growth temperature (≥ 900 °C) and the presence of polymers, fabrication is commonly based on a manual transfer process of pre-grown graphene sheets, which causes reliability issues. In this paper, we present CVD-based multilayer graphene electrodes fabricated using a wafer-scale transfer-free process for use in optically transparent and MRI-compatible neural interfaces. Our fabricated electrodes feature very low impedances which are comparable to those of noble metal electrodes of the same size and geometry. They also exhibit the highest charge storage capacity (CSC) reported to date among all previously fabricated CVD graphene electrodes. Our graphene electrodes did not reveal any photo-induced artifact during 10-Hz light pulse illumination. Additionally, we show here, for the first time, that CVD graphene electrodes do not cause any image artifact in a 3T MRI scanner. These results demonstrate that multilayer graphene electrodes are excellent candidates for the next generation of neural interfaces and can substitute the standard conventional metal electrodes. Our fabricated graphene electrodes enable multimodal neural recording, electrical and optogenetic stimulation, while allowing for optical imaging, as well as, artifact-free MRI studies.


Mask design
In the Mo/graphene mask, holes are designed to promote a better adhesion of the metal layer (Al (1%Si) / Ti) that will be applied on top of graphene with the layer underneath (SiOx).
In the Al/metal mask, the metal layer on top of the electrode is smaller such that it can easily be removed from graphene when needed. Moreover, once the metal is being removed, the graphene electrode area should not be fully exposed but protected against possible delamination, at its outermost ring, by the polymer on top (smaller opening on in the polymer opening mask for the electrodes). The metal layer on the contact pads of graphene is slightly larger than graphene contact pads. The reason is the adhesion needed to keep the metal layer on top of graphene until the final steps of microfabrication process.

(e)
Additionally, in the polymer opening mask, the dimensions of the openings for the contact pads are the same as for the graphene pads, to ensure that graphene pads are completely covered by the metal layer. This Al metal layer on the contact pad was also designed for soldering wires to the contact pads. However, the openings for the electrodes are smaller than the metal to avoid damaging the graphene layer in case of misalignment of the mask during the lithography steps.

Graphene electrodes on Si
Pt and Au electrodes on Si

Sheet resistance measurement
Van der Pauw structures for four-point probe measurements  Four-point probe measurements were performed with a Cascade Microtech probe station. The four probes were positioned on the Ag ink dots on the extremities of the Van der Pauw structures. The current was forced between Probes 1 and 2 (I1,2), by setting the Probe 1 potential from -1 V to 1 V with 8 mV steps and Probe 2 acting as ground. The voltage was measured between Probes 3 and 4 (V3,4). The sheet resistance (Rsh) was calculated using the equation below for each point in Ω/sq unit, which were then averaged, excluding the values around I = 0.

Graphene transfer process
The method used for graphene transfer started with a full wafer with graphene grown on non-patterned Mo. The wafer was then manually diced into roughly 2 cm 2 pieces. These were then placed in a beaker where H2O2 was added just until the height of the Si piece with graphene. Mo then started to etch from the sides towards the centre, as illustrated in Fig. S5 (a). After all the Mo was etched, the graphene was separated from the silicon piece and floated on the surface of the liquid while the silicon fell to the bottom of the beaker. DI water was then added very gently with a pipette until the level of the liquid rose about 2 cm. The floating graphene was then scooped on the glass slide. One drop of Triton X100 was added to 1 litre of DI water and 50 ml of triton-water solution was added to the H2O2 in the beaker to reduce surface tension. The resulting glass slides with transferred graphene are shown in Fig. S5 (b).

Figure of merit (FOM)
To evaluate the quality of a transparent conductive film (TCF), the figure of merit (FOM) is calculated based on optical transmittance (T) at 550 nm wavelength and sheet resistance (Rsh).
Films with low Rsh at high optical transmittance show higher performance as a TCF, therefore, a higher FOM corresponds with better TCF performance. The most widely used FOM is defined as follows: The FOM calculated based on this equation can be used to evaluate all films with different thicknesses, synthesised from different methods and materials.
The reported values in this work are comparable with the result from graphene electrodes manufactured using a CVD process and also higher than the theoretical value (2.55) calculated by the same equation for an undoped monolayer graphene. However, the calculated FOM is as expected a lot lower than the one of ITO (60 for 40 nm).

Electrical impedance spectroscopy (EIS)
The EIS data for graphene electrodes using 40-and 60-min graphene growth recipes are shown in the Fig. S6 (a, b), respectively. The impedance of three 40-min graphene electrodes at 1kHz

Equivalent circuit model
The equation used for ZCPE in the equivalent circuit model is: where j is the unit imaginary number for which it holds that j 2 = −1, ω is the angular frequency being 2π times the frequency of the AC signal (ω= 2πf), Q is a measure of the magnitude of  ZCPE, and n is a constant in the range between 0 to 1. When n = 1, ZCPE is a purely capacitive impedance element, and Q is capacitance; when n = 0, ZCPE is a purely resistive element, and Q is its conductance, the reciprocal of its resistance. For practical electrode-electrolyte interfaces, the ZCPE is used instead of a pure capacitance, accounting for the non-ideal capacitive behavior of the electrochemical double layer.
The equation used for ZWB in the equivalent circuit model is: Where WB is the finite-length Warburg coefficient and B is: Where D is the diffusion coefficient and d is the thickness of the diffusion layer.

Cyclic voltammetry (CV)
The CV scan started at 0 V, then the potential increased until the upper limit. Next, the potential decreased to the lower limit and after that returned back to 0 V. The scan was repeated 3 times to stabilise the signal, and the third scan was used in the calculation of the CSC. The CSC is calculated based on the time integral of the CV curve. The calculated charge was then divided by the electrode surface area (68320μm 2 ) to obtain the charge density. Finally, the CSC was expressed in μC/cm 2 .

Atomic force microscopy (AFM)
Since any surface roughness greatly increases the CSC due to an increase in the electrochemical surface area of the electrode, the prepared graphene samples were characterized by atomic force microscopy (AFM). AFM was conducted using an NTEGRA Spectra system at ambient conditions. The morphology was captured in the tapping mode with a NSG01 probe. The surface roughness was measured across the 50 × 50 μm 2 areas, calculated as the root-mean square of the height distributions, and then averaged. The phase lag of the AFM probes was measured simultaneously with the topography to achieve better contrast of small topographic features. High-quality images were processed in the standard way using Gwyddion, applying polynomial correction of the background. Then, surface characterization was used to determine the surface roughness.

EDX measurement
For this measurement, Mo is deposited on a Si wafer with a pre-deposited SiO2 layer. Next, graphene is grown on Mo catalyst. Then, the Mo is etched using a simple wet etching process (using H2O2) as was also used in the paper. To verify that there is no Mo residue energy dispersive X-ray (EDX) (FEI XL30 SFEG, w. EDAX Octane Plus detector) analysis was performed. During the EDX measurement the electrode surface after Mo removal was focused using 15 kV electron beam and a spot size of 6. The elemental mapping results acquired after 92 minutes represented in Fig. S9 show the presence of Si, O, C, and Mo and the corresponding spectrum. The indicated areas with graphene and without graphene in the SEM image matches with the C map.
Details of the atomic and weight percent of each element are listed in the inset Table   represented on the right side of the diagram.